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Alignment here means that the size of the range passed into callback is a multiple of the alignment value (which has to be a power of two). This can help with performance when loops in the callback are are unrolled and/or vectorized. Otherwise, it can potentially reduce performance by splitting work into more unequally sized chunks. For example, chunk sizes might be 4 and 8 instead of 6 and 6 when alignment is 4.